Conventionally, biological wastewater treatments of organic matter-containing wastewater containing organic matter or the like have used the activated sludge method that utilizes an aggregate of microbes (aerobic biological sludge) known as a flock. However, in the activated sludge method, when separating the flock (aerobic biological sludge) and the treated water in the settling tank, because the settling rate of the flock is slow, the surface area of the settling tank must sometimes be increased substantially. Further, the treatment rate of the activated sludge method depends on the sludge concentration in the biological treatment tank, and therefore the treatment rate can be increased by increasing the sludge concentration, but if the sludge concentration is increased to a value within a range from 1,500 to 5,000 mg/L, or an even higher value, then solid-liquid separation becomes difficult due to bulking or the like within the settling tank, meaning treatment cannot be maintained in some cases.
On the other hand, anaerobic biological treatments generally utilize aggregates (anaerobic biological sludge) in particulate form known as granules, which contain densely packed microbes. Granules exhibit extremely fast settling rates, and because the microbes are densely packed, the sludge concentration in the biological treatment tank can be increased, and a high-speed treatment of the wastewater can be achieved. However, anaerobic treatments sometimes suffer problems, including restrictions on the types of wastewaters that can be treated compared with aerobic treatments (the activated sludge method), and the requirement to hold the temperature of the treatment water at about 30 to 35° C. Further, if only an anaerobic treatment is used, then the water quality of the treated water tends to be poor, and in those cases where the treatment water is to be discharged into a river or the like, a separate aerobic treatment such as a biological sludge treatment is sometimes required.
In recent years, it has become clear that by performing treatment using a semibatch treatment device in which the wastewater is introduced intermittently into the reaction tank, and shortening the biological sludge settling time, granulated biological sludges having good settling properties can be formed not only with anaerobic sludges, but also with aerobic sludges (for example, see Patent Documents 1 to 4). By granulating an aerobic sludge, the average particle size can be increased to 0.2 mm or greater, and the settling rate can be increased to 5 m/h or greater. In a semibatch treatment device, treatment is performed in a single biological treatment tank via four steps, namely (1) wastewater introduction, (2) biological treatment of the treatment target substances, (3) settling of the biological sludge, and (4) discharge of the treated water. By forming the type of granulated aerobic biological sludge with good settling properties described above, the sludge concentration in the tank can be maintained at a high concentration, enabling a high-speed treatment to be achieved.
One method that has been proposed for accelerating the granulation is a method in which the settling time for the aerobic granules is shortened, thereby proactively discharging sludge having a slow settling rate from the system, but in this method, because the amount of discharged microbes fluctuates depending on changes in the sludge settling properties, forming the aerobic granules in a stable manner is sometimes difficult. Further, in the case of wastewaters such as sewage having a low BOD concentration of about 80 to 200 mg/L, another problem arises in that the aerobic granules tend to be difficult to form, even when using a semibatch reactor.